The present invention relates to a multi-output switching power source apparatus for supplying stabilized DC voltages to a plurality of loads in industrial and consumer electronic appliances. As this kind of multi-output switching power source apparatus, the multi-output switching power source apparatus disclosed in Japanese Patent Publication No. 2803186 discloses is available.
FIG. 9 is a circuit diagram of the multi-output switching power source apparatus disclosed in Japanese Patent Publication No. 2803186. The reference numerals in the Japanese Patent Publication No. 2803186 are changed so as to be matched with those in the explanations of the present invention. FIG. 10 shows operation waveforms at various portions in the circuit shown in FIG. 9.
The operation of the conventional multi-output switching power source apparatus will be described below by using FIG. 9 and FIG. 10.
First, when a main switching circuit 202 is ON, an input voltage Ei is applied to the primary winding 231 of a transformer 203. At this time, a current Id flows through the primary winding 231. When the main switching circuit 202 turns OFF at time t1, a current Is1 flows from an output winding 233 via a diode 241, and a current Is2 flows from an output winding 234 via a diode 261. A switching device 260 connected in parallel with the diode 261 is turned ON by a drive pulse signal Vg6 from a control circuit 213 during a period between time t1 and time t2. The current Is2 becomes zero at time t2 but flows continuously in the opposite direction via the switching device 260. Hence, an output voltage Eo2 is applied to the output winding 234. On the other hand, when the current Is1 becomes zero, the diode 241 turns OFF, and the current Is1 stops flowing.
The control circuit 213 controls the ON period of the switching device 260. When the switching device 260 turns OFF at time t3, the voltage across each winding of the transformer 203 is reversed. At this time, a diode 221 conducts on the primary side. A drive circuit 209 detects that the voltage across a drive winding 232 has been reversed, and outputs a drive pulse signal Vg2 for turning on the switching device 220 of the main switching circuit 202. The current Id flowing through the main switching circuit 202 flows so as to regenerate electric power for a DC power source 1.
This current Id becomes zero at time t4 as shown at a part (b) of FIG. 10. After time t4, the current Id flows from the DC power source 1 to the primary winding 231 and the switching device 220. At time t5, the switching device 220 turns OFF, and the above-mentioned operation after time t1 is repeated.
When the number of turns of the primary winding 231 is N31, when the number of turns of the output winding 233 is N33, when the number of turns of the output winding 234 is N34, and when the ON period and the OFF period of the main switching circuit 202 are Ton and Toff respectively in the above-mentioned operation, output voltages Eo1 and Eo2 are represented by the following expressions (1) and (2) respectively:
Eo1≈(N33/N31)xc2x7(Ton/Toff)xc2x7Eixe2x80x83xe2x80x83(1), and
Eo2≈(N34/N31)xc2x7(Ton/Toff)xc2x7Eixe2x80x83xe2x80x83(2).
The relationships shown in the expressions (1) and (2) are similar to the relationships between the input and output voltages of an ordinary flyback converter. In the case of the conventional multi-output switching power source apparatus shown in FIG. 9, the drive circuit 209 sets the ON period Ton, and the control circuit 213 adjusts the OFF period Toff so as to stabilize the output voltage Eo1.
Generally in a multi-output switching power source apparatus, the output voltages of unstabilized outputs, other than outputs controlled directly so as to be stabilized, fluctuate depending on input and output conditions. This kind of voltage fluctuation is caused by induced voltages due to leak inductances of a transformer and voltage drops due to line impedances. In the case of the output voltage (Eo2) not controlled so as to be stabilized for example, when its output current (Io2) increases, a voltage drop due to a line impedance increases, and the output voltage (Eo2) lowers. On the other hand, in the case of the output voltage (Eo1) controlled so as to be stabilized, when its output current (Io1) increases, the ON period (Ton) of the main switching circuit 2 becomes long, and the output voltage (Eo2) rises. The output voltage (Eo2) fluctuates up and down as described above to stabilize the output voltage (Eo1) in spite of the voltage drop due to the line impedance. In particular, when the output current (Io2) is very small, an induced voltage due to a leak inductance, which is superimposed on the winding voltage of the transformer 3, raises the output voltage (Eo2) further.
In the conventional multi-output switching power source apparatus shown in FIG. 9, however, the output current Io1 serving as a stabilized output is large; even if the output current Io2 serving as an unstabilized output is zero, the above-mentioned rise of the output voltage Eo2 does not occur. This is because the current Is2 flows during the whole range of the OFF period Toff regardless of input and output conditions, whereby a condition wherein the output current Io2 is equivalently large is maintained. Furthermore, the induced voltage due to the leak inductance, which is superimposed on the output winding 34, is controlled, and the voltage drop due to the line impedance is not affected by the output current Io2.
However, in the conventional multi-output switching power source apparatus shown in FIG. 9, when the output current Io1 is small, the voltage drop due to the line impedance is also small, and the ON period Ton becomes short. In this case, the output voltage Eo2 rises, thereby causing a problem.
To cope with the diversification of loads in industrial and consumer electronic appliances, multi-output switching power source apparatuses capable of stably supplying a plurality of output voltages in all conditions are demanded. The present invention is intended to provide a multi-output switching power source apparatus capable of controlling the fluctuations of output voltages in any load conditions excluding overloads.
A multi-output switching power source apparatus in accordance with the present invention comprises:
a main switching circuit for converting the voltage of a DC power source into an AC voltage and for inputting the voltage to the primary winding of a transformer,
the transformer having a plurality of output windings including the primary winding,
a plurality of secondary switching circuits to which AC voltages induced across the plurality of output windings are input and which turn ON/OFF complementarily with the main switching circuit,
a plurality of smoothing circuits connected to the plurality of secondary switching circuits respectively, and
a control drive circuit for turning ON each of the secondary switching circuits after the main switching circuit is turned OFF, for turning ON the main switching circuit after each of the secondary switching circuits is turned OFF, for detecting one of a plurality of DC output voltages and for adjusting the ON period of the main switching circuit and the ON period of each of the secondary switching circuits to stabilize the DC output voltage.
In the multi-output switching power source apparatus in accordance with the present invention configured as described above, each of the secondary switching circuits turns ON during the OFF period of the main switching circuit, whereby the outputs are short-circuited via the transformer. For this reason, factors for fluctuating each output voltage can be limited to a voltage drop due to the line impedance of each output. Hence, the present invention can control the fluctuations of the output voltages in any load conditions including electric power supply from a load but excluding overloads.
A multi-output switching power source apparatus in accordance with another aspect of the present invention comprises:
a DC power source,
a transformer having a primary winding and a plurality of output windings,
a main switching circuit for converting the voltage of the DC power source into an AC voltage by repeating ON/OFF operation and for inputting the voltage to the primary winding of the transformer,
first to nth (n: a natural number of 2 or more) secondary switching circuits connected to the first to nth output windings of the plurality of output windings respectively and turning ON/OFF complementarily with the main switching circuit in order to rectify AC voltages induced across the first to nth output windings,
first to nth smoothing circuits for smoothing voltages rectified by the first to nth secondary switching circuits and for supplying first to nth DC output voltages to first to nth loads, and
a control drive circuit for turning ON the first to nth secondary switching circuits after the main switching circuit is turned OFF, for turning ON the main switching circuit after the first to nth secondary switching circuits are turned OFF, for detecting one of the first to nth DC output voltages and for adjusting the ON period of the main switching circuit and the ON periods of the first to nth secondary switching circuits to stabilize the DC output voltage. The multi-output switching power source apparatus in accordance with the present invention configured as described above can control the fluctuations of the output voltages against all load conditions excluding overloads and can transfer electric power among the outputs via the transformer; when electric power is supplied from one load side, the electric power can be supplied to the other load side via the transformer.
A multi-output switching power source apparatus in accordance with still another aspect of the present invention comprises:
a DC power source,
a transformer having a primary winding and at least one output winding,
a main switching circuit for converting the voltage of the DC power source into an AC voltage by repeating ON/OFF operation and for inputting the voltage to the primary winding of the transformer,
a first secondary switching circuit connected to the primary winding and turning ON/OFF complementarily with the main switching circuit in order to rectify an AC voltage induced across the primary winding,
second to (n+1)th (n: a natural number) secondary switching circuits connected to the first to nth output windings of the output windings and turning ON/OFF complementarily with the main switching circuit in order to rectify AC voltages induced across the first to nth output windings,
first to (n+1)th smoothing circuits for smoothing voltages rectified by the first to (n+1)th secondary switching circuits and for supplying first to (n+1)th DC output voltages to first to (n+1)th loads, and
a control drive circuit for turning ON the first to (n+1)th secondary switching circuits after the main switching circuit is turned OFF, for turning ON the main switching circuit after the first to (n+1)th secondary switching circuits are turned OFF, for detecting one of the first to (n+1)th DC output voltages and for adjusting the ON period of the main switching circuit and the ON periods of the first to (n+1)th secondary switching circuits to stabilize the DC output voltage. The multi-output switching power source apparatus in accordance with the present invention configured as described above can control the fluctuations of the output voltages against all load conditions excluding overloads and can transfer electric power among the outputs via the transformer; when electric power is supplied from one load side, the electric power can be supplied to the other load side via the transformer.